Influence of magnetic fields on luminescence involving triplet excitons

Several processes involving triplet excitons Iead to magnetic-field sensitive luminescence in organic crystals such as anthracene and related compounds. Small magnetic fields (Zeeman energies ~ thermal energies) at room temperature give rise to observable effects. The firstdass of phenomena is subject to fine structure modulation (FSM) and requires fields of lOOs to lOOOs of oersteds. This class includes triplet-triplet fusion leading to higher energy singlets which emit delayed tluorescence and singlet exciton fission (or inverse of triplet-triplet fusion) leading to a decrease in prompt fluorescence yield. The second class of phenomena is subject to hyperfine modulation (HFM) and fields of only lOs of oersteds are required. The key example is dye-sensitized delayed fluorescence involving an intermediate charge-transfer state. All the above phenomena involve non-equilibrium processes subject to spin restrictions. The fine structure interaction in FSM and the hyperfine interaction in HFM Iead to spin 'precessional' motions which circumvent these restrictions. The external field competes with the internal magnetic interactions in affecting these spin motions.